Parkinson's disease (PD) is a worldwide degenerative and progressive condition characterized by tremor and rigidity. PD is caused by the loss of dopaminergic neurons in the substancia nigra pars compacta, which innervate the striatum and provide it with dopamine (DA). Embryonic stem (ES) cells are known to replicate and to differentiate to several nerve cells, including DA neurons. In fact, dopaminergic cells generated from mouse ES cells can reverse the behavioral abnormalities present in experimental models of PD. To have experimental models has been very useful to study the therapeutic value of different approaches. In this proposal, we want to know what are the modifications in DA neuron markers induced by grafting ES cell- derived-DA neurons in the striatum or the substantia nigra. We also want to establish if cell therapy with mouse ES cell-derived progeny can reconstitute the natural nigro-striatal pathway by. To answer this, we will graft DA neurons in the substancia nigra at different stages of development, from fetuses to adults. Since it has been shown that adult brains are a poor permissive environment for axonal growth, we will use strategies that could promote regeneration of the lost DA trait. Previous results show that grafting ES cells in the striatum can partially restore DA levels. However, a higher basal DA concentration was found in grafted animals. Therefore, we will conduct microdialysis experiments after grafting parkinsonian rats with mutant ES cells to find out if DA concentration in vivo is regulated by mechanisms of neuronal nature (i.e. DA synaptic release and DA uptake). We want to differentiate ES cells null for synaptic release, and devoid of DA transporter activity to DA neurons, and graft them to study the behavioral recovery provided by these genetically modified mouse ES cells. All these approaches can help us understand the basic Biology of stem cell, differentiation, and test if recovery is achieved, to consider translating them to the clinic in the long term.